1. Field of the Invention
This invention relates to a display device for displaying photography modes and photography information within a view finder of a photographic camera and more particularly to a display system which includes a display device disposed within a view finder and another display device disposed without the view finder. The display lights up or is put out depending on to the photography mode selected.
2. Description of the Prior Art
Conventional display devices of this kind include devices of the type having dot light emitting diodes (hereinafter called dot LED's for short) and a display film in combination outside of a view finder visual field to make a display by illuminating the display film with the dot LED's in a manner shown in FIGS. 1, 2 and 3 of the accompanying drawings. Referring to FIG. 1, a light from an object to be photographed, which comes through a photo-taking lens 1, is reflected by a turning mirror 2 to be imaged on a focus plate 3. The object image thus formed on the focus plate 3 is reflected by a penta-Dach prism 4 observable is a view finder field image as well known. Reference numeral 6 identifies a Fresnel lens for converging and reference numeral 7 identifies a light receiving photo sensitive element. They are arranged above an eyepiece 5 to confront the exit surface of the penta-Dach prism 4 as a light receiving system for light measurement. An information display member A is provided with a display film 8 and is below the penta-Dach prism 4 with the display film 8 opposed to a bottom face 4a of the penta-Dach prism 4. FIG. 2 is an enlarged exploded view of the above information display member A. The display film 8 is provided with, for example, information display marks such as photography mode display marks 8a and 8b and aperture value marks 8c, 8d, 8e, 8f, etc. Dot LED's 11a, 11b, 11c, 11d, 11e, 11f, etc. are secured onto a substrate 12 in positions corresponding respectively to the information display marks 8a, 8b, 8c, 8d, 8e, 8f, etc. Between the display film 8 and the substrate 12 are interposed, one after another, a diffusing tape 9 which has each of the information display marks 8a-8f evenly illuminated by the light from the corresponding LED's 11a-11f and a light shielding plate 10 which prevents the light of each of the LED's 11a-11f from leaking to any of the information display marks 8a-8f that differ from the corresponding one. FIG. 3 shows a view finder field. In the illustrated example, the information display mark 8e of the display film 8, which indicates an aperture value "16" of the photo-taking lens 1, is illuminated by a light emitted from the corresponding LED 11e via the combined arrangement of the light shielding plate 10 and the diffusing tape 9, in such a manner that the mark 8e alone appears on one side of the photo-taking visual field F as display information I.
The view finder display device of this type necessitates the provision of terminals and lead wires around the display part as power supply terminals and lead wires are indispensable to the display part. Therefore, the substrate 12 of the LED's 11a-11f includes a large unused area therein.
In view of this problem, the use of a seven-segment LED for a view finder display device in a camera has been proposed. FIG. 4 shows, by way of example, the essential parts of that type of view finder display device. The illustration of FIG. 4 includes a focus plate 21; a condenser lens 22; and a penta-Dach prism 23. An information source 24 is in a lower part behind the exit surface of the penta-Dach prism 23. A seven-segment LED 24b is attached to the surface of a substrate 24a. Numerals and letters indicative of shutter time values, aperture values and manual and automatic operation modes are formed, one after another, by means of the seven-segment LED 24b perpendicular to the paper surface of the drawing. Furthermore, the part of the substrate 24a other than the seven-segment LED 24b is an unused area 24c. The surface of the substrate 24a is covered with a plastic cover 24d. An information light introducing small prism 25 is disposed between the bottom surface (an incident surface) 23a of the penta-Dach prism 23 and the condenser lens 22 in front of the information source 24. The small prism 25 has an incident surface 25a, a reflection surface 25b and an exit surface 25c. A mask plate 26 is on the bottom surface 23a of the penta-Dach prism 23 to define a photo-taking visual field and an information display picture plane.
FIG. 5 shows a view finder visual field obtained by the display device of FIG. 4. Photography information 27 appears at a lower part on the lower outside of the photo-taking field F. The photography information 27 consists of shutter time information 27a, aperture value information 27b and automatic/manual selection information 27c. In this illustration, for example, the shutter time information 27a indicates "1000" (1/1000 sec), the aperture value information 27b "5.6" and the selection information 27c "M" (for manual).
The view finder display device of this type is extremely expensive as it necessitates the use of the seven-segment LED's 24b for many figures and the small prism 25 for light introduction. Compared with the penta prism 4, 23 generally used in conventional devices, use of a larger penta-prism 4, 23 is necessary since the lower visual field must be enlarged.
The present invention is directed to the solution of the above problems of conventional display devices. The invention aims to provide a view finder display device for a camera in which the display device of the type using an optical system as shown in FIG. 1 is modified; and a film display member illuminated by the dot LED's 11a-11f is on the same substrate 12 as that of a seven-segment LED 24b, so that a device capable of indicating many kinds of information 27 can be obtained at reduced size and cost.
Meanwhile, for driving the LED's 11a-11f, at constant current circuit which is shown in FIG. 6 or 7 has been employed. In the circuit shown in FIG. 6, a constant voltage of a constant voltage circuit 31 is impressed on the non-inversion input terminal of an operational amplifier 32, as well known, when voltage is supplied between a power source Vcc and a ground GND. Therefore, a negative feedback action brought about by the operational amplifier 32 and an NPN transistor 36 causes the potential of the inversion input terminal of the operational amplifier 32 to become equal to the constant voltage of the constant voltage circuit 31. As a result, a constant current flows to a resistor 35. When the current amplification rate (hfe) of the NPN transistor 36 is sufficiently high, the collector current and the emitter current of the NPN transistor 36 become equal. This also causes a constant current to flow to an LED 34. The LED 34 thus has a constant current flow thereto regardless of the voltage between the power source Vcc and the ground GND. Therefore, a display within the view finder remains stable. However, the operational amplifier 32 includes many transistors and thus has a great number of elements included therein, resulting in a complex structural arrangement. Furthermore, assuming that four LED's 34 are used for the display, for example, in order to equalize the brightness of these LED's 34 by individually adjusting currents for them according to their various characteristics, a total of four sets of the circuit of FIG. 6 is necessary with the exception of the constant voltage circuit 31 (the current value is adjusted by means of the resistor 35). The conventional circuit arrangement thus results in a great number of elements. Then, an attempt to reduce the number of elements has resulted in the circuit shown in FIG. 7. In this circuit arrangement, when a voltage is supplied between the power source Vcc and the ground GND, a constant voltage of a constant voltage circuit 41 is impressed on the base of an NPN transistor 42. The emitter of the NPN transistor 42 then produces a voltage which is obtained by subtracting a voltage between the base and emitter of the NPN transistor 42 from the constant voltage of the constant voltage circuit 41. A resistor 43, thus, approximately has constant voltages applied to the two terminals thereof. As a result, a nearly constant current flows to the resistor 43. This constant current also flows to a PNP transistor 44 which is diode connected. PNP transistors 45 and 46 have their bases connected in common with the PNP transistor 44. Therefore, if the resistance values of resistors 47 and 48 are zero, the collector currents of the PNP transistors 45 and 46 become equal to that of the PNP transistor 44. Therefore, it would be possible to adjust the uneven characteristics of LED's 49 and 50 by adjusting the value of the resistors 47 and 48 causing the LED's 49 and 50 to emit a light of even brightness. However, the voltages between the collectors and emitters of the PNP transistors 45 and 46 vary with the voltage between the power source Vcc and the ground GND. Then, the Early effect (base width modulating effect) of the transistors causes the collector currents of the PNP transistors 45 and 46 to change with the power supply voltage (or the voltage between the power source Vcc and the ground GND). The camera uses a battery the voltage of which decreases with the length of time in service. Therefore, variations in the brightness of display LED's present great inconvenience.
A first object of this invention is to provide a display device which can be compactly arranged with a small number of parts for displaying a selected photography operation mode and exposure information computed on the basis of information on a preset value.
A second object of this invention is to provide a display device wherein shutter time information and aperture value information are computed according to the photography operation mode selected; the selected mode and the computed information are displayed; and one information display can be shifted to another depending on the result of computation.
A third object of this invention is to provide a display device for display photography information for a camera capable of switching from one photography operation mode to another, wherein exposure information is displayed in a symbolized form when the number of numeral places of computed exposure information exceeds the number of places that can be displayed.
A fourth object of this invention is to provide a display device consisting of a view finder display device and an external display device which are arranged such that the illumination of the view finder display device is put out and exposure information is displayed by the external display device when an eyepiece shutter is closed.
These and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.